This invention concerns a method to operate a fluidized bed incinerator which incinerates waste containing solid carbon, such as sewage sludge, municipal garbage or industrial waste, and the incinerator employing this method. More specifically, it concerns a method to operate a fluidized bed incinerator which incinerates waste with a high moisture content, such as sewage sludge, and the incinerator employing this method.
Fluidized bed incinerators can be divided into two types: those using fluidized beds of air bubbles, which are commonly employed to incinerate garbage and evaporated sewage sludge, and those using circulating fluidized beds, which are commonly employed in coal-burning boilers which generate electrical power and incinerators which burn a mixture of waste and fuel.
Fluidized bed incinerators employing air bubbles work as follows. When the velocity of the gas exceeds the speed at which the particles comprising the medium of flow become a fluid, air bubbles begin to form on the floor of the fluidized bed. These bubbles agitate the medium of flow, causing the interior of the bed to achieve an ebullient state, in which the fuel is combusted.
In circulating fluidized bed incinerators, the velocity of the aforesaid gas is forced to exceed the terminal velocity of the particles comprising the medium of flow. As the gas and the particles are vigorously mixed, the particles are entrained on the gas and dispersed and combusted above the fluidized bed. The dispersed particles are collected by a separating device such as a cyclone and recirculated in the incinerator.
These two types of fluidized bed incinerators account for most of the incinerators in use. Both are suitable for combusting low-quality fuel or waste. Most sewage sludge is processed in a fluidized bed incinerator, and municipal garbage and industrial waste tend to be burned in an incinerator connected in series with a stoker.
The configuration of the aforesaid air bubble-type fluidized bed incinerator is shown in FIG. 18. The bottom of a vertical cylindrical tower is filled with a quantity of sand 50a, the fluidizing medium. This sand forms bed region 50 (the bubbling region or the dense region). A fluidizing gas is injected through air inlet 53 and thereafter forced uniformly through dispersion devices 52, dispersion tubes feeding into the bottom of the bed. The velocity of the gas, which is the flow velocity at which the said gas is injected, is increased until it exceeds the speed at which the aforesaid fluidizing medium becomes a fluid. Air bubbles 50b form in the aforesaid fluidizing medium, agitating and fluidizing it, and causing its surface to assume an ebullient state.
The sludge to be incinerated is loaded into the furnace via sludge inlet 55, which is above the aforesaid bed region 50, now in an ebullient state. At the same time, an accelerant is loaded via inlet 54 and combusted. After the solid component of the sludge is combusted in bed region 50, its volatile component is combusted in freeboard 56, the space above bed region 50. The exhaust gas from the said combustion is released through exhaust vent 57 on the top of the tower.
In an air bubble-type fluidized bed incinerator, waste such as raw garbage or sludge is combusted through the following process.
1) The air used to create a fluid is injected via gas dispersion devices 52 at the start of combustion. The sand is heated by a burner from the top layer down. As its temperature rises, the bed is fluidized by air bubbles.
2) Next, the garbage to be incinerated is loaded into the chamber. If the heat value of the garbage is too low, an accelerant is introduced to maintain the interior of the bed at the proper temperature.
3) After combustion has begun, the air heated by the exhaust gas is used as the aforesaid fluidizing gas. The garbage in the chamber is vigorously mixed and fluidized with the heated sand in the bed region. After a short time, part of it is gasified by dry distillation, and the remaining solids are combusted.
4) The uncombusted gases and the volatile or light portions of the garbage are conducted to freeboard 56, the area above the fluidized bed, and there combusted.
When sewage sludge is incinerated in the aforesaid air bubble-type fluidized bed incinerator, the rate of combustion in the furnace is 60 to 80% in the fluidized bed, but it climbs to nearly 100% in the area of the freeboard.
Thus the combustion load of freeboard 56 is 20 to 40%, and the temperature of the freeboard is approximately 150xc2x0 C. higher than that of the fluidized bed. Since the combustion energy required to incinerate raw garbage or sludge is likely to vary, parts of the freeboard may become too hot.
In an air bubble-type fluidized bed incinerator, the air heated by the exhaust gases to approximately 650xc2x0 C. is reused in order to conserve energy and minimize pollution. To prevent harmful exhaust, the temperature at the vent of the incinerator must be regulated so that the average temperature of the uncombusted gases (mainly CO, dioxin and cyanogen) is around 850xc2x0 C.
In order to maintain the sand bed fluidized by the medium at an appropriate average temperature, say between 700, and 750xc2x0 C., the moisture load at the floor of the furnace must be less than 250 to 280 kg/m2h. Because of the limitations of the equipment, the aforesaid velocity of the gas must be at least 0.5 m/s (to maintain stable bubbling, it must be 0.5 to 1.5 m/s). Thus to incinerate waste with a high water content, such as sewage sludge, the floor of the furnace is made larger than is necessary for combustion, and more air is supplied than is actually needed for combustion. More exhaust gas is produced, and the extra air is wasted.
In many cases, the relative density of the substance to be incinerated is equal to or less than that of the fluidized bed. If the substance is less dense than the bed, when it is loaded into the chamber via the freeboard it will float on the surface of the fluidized sand on the very top of bubbling region, and the temperature within that region will not be conducive to effective combustion.
Sewage sludge has a relative density of approximately 0.8 t/m3. When it is loaded into the furnace, however, its moisture component immediately evaporates, leaving it with a density of 0.3 to 0.6 t/m3. Assuming silica with a relative density of 1.5 t/m3 is used as the fluidizing medium, it will attain a relative density of 1.0 t/m3 also assuming that the bed expands by a factor of 1.5.
In a case like this, where the substance to be incinerated is relatively light, it will float on the surface of the sand in the bubbling region even if it is loaded from the freeboard. The combustion of the substance will be limited to the top layer and will not extend to the interior of the bed. This imposes limitations on the maximum load which are not present when combustion can be extended effectively to the entire lower portion of the bed, including the bubbling region in the lower half of the air bubble bed and the dense layer below it.
Moreover, if combustion is achieved only in the upper portion of the aforesaid sand bed, the volatile component of the substance to be burned will be propelled through the splash region above the bed and combusted in the freeboard. There will be more combustion in the freeboard, which has a low thermal capacity, and less in the region which contains the dense layer of sand with its high thermal capacity. As a result, the temperature in the furnace will be unstable.
Another problem which can occur is that the waste product which falls onto the sand on top of the aforesaid bubbling region may not break up effectively. This results in some portions remaining uncombusted and leads to improper fluidization.
Also, waste matter like raw garbage and sewage sludge contains a high volume of volatile components. Since these sublimate, they are combusted in the freeboard. This causes the temperature of the exhaust gases to be too high.
In particular, if the temperature of the sand in the fluidized bed drops below 750xc2x0 C., the combustion rate in the bed will decrease, increasing the prospect of unstable combustion. Thus the temperature of the sand must be kept at 750xc2x0 C. or higher. When the volatile component is combusted in the aforesaid freeboard, it cannot contribute to maintaining the temperature of the sand. This necessitates the addition of a great deal of accelerant.
As we have noted, prior art air bubble-type fluidized bed incinerators experience problems due to the differing fuel quality of different waste substances. If the waste contains a high proportion of volatile components, the temperature will spike in the freeboard. If the waste contains a great deal of moisture, the temperature of the sand will drop. There was no effective way to address these problems in the prior art.
In addition, prior art techniques could not mitigate the problem of temperature fluctuation in the freeboard caused by varying fuel quality in different parts of the waste material.
Since the temperature of the sand was likely to drop when a waste substance with a high moisture content like sludge was combusted in the fluidized bed, an accelerant was used to maintain a high temperature. However, since some or in some cases almost all of the accelerant would immediately sublimate, it would combust in the freeboard without contributing to the temperature of the sand. The accelerant was thus combusted to no purpose, which had a deleterious effect on the fuel cost.
To solve the aforesaid problems associated with air bubble-type fluidized bed incinerators, the present applicants investigated how to mitigate the overheating of the freeboard and how to elevate the density of the suspension in the freeboard so as to maintain it at a high thermal capacity in order to prevent load fluctuations, particularly those due to the varying quality of the substance to be burned. We also studied ways to circulate the heat from the combustion in the aforesaid freeboard into the region of the fluidized bed. In the course of these investigations, we developed the following techniques.
In the following section we shall discuss the techniques we developed, following the order of our investigations.
To recirculate the heat from the combustion in the aforesaid freeboard back to the fluidized bed, we might consider the use of a circulating fluidized bed. But a circulating bed lacks a distinct dense layer (dense bed) in its lower portion, so its capacity to absorb load fluctuations is negligible, and the characteristics of the exhaust gases are likely to be unstable.
One approach resulting in a fluidized bed incinerator with a distinct dense layer and which employs a method to entrain and recirculate the fluidizing medium is to use a medium which consists of particles of both a finer and a coarser grain. The finer particles form an entraining fluidized bed, and the coarser particles form a heavy fluidized bed. By combining the two sorts of beds, one achieves a furnace which can control the combustion of pulverized coal. The design of such a furnace is disclosed in Japanese Patent Publication (Koukoku) 60-21769.
Overlaying an entraining fluidized bed of fine particles on a dense fluidized bed of coarser particles creates a high-density bed with two distinct temperature regions in its upper and lower halves. The design for a furnace using such a bed, which entails both combusting and gasifying high-sulfur coal, is disclosed in Japanese Patent Publication (Koukoku) 63-2651.
Both of the aforesaid approaches involve a fluidized bed consisting of an entraining bed made of fine particles which is superimposed on a heavy bed consisting of coarse particles. Since these coarse particles, the fluidizing medium in the heavy bed, experience significant abrasion, they must be replenished frequently, which complicates the maintenance of the furnace. Also, the use of the aforesaid coarse particles which are prone to abrasion results in a loss of stability due to variations of the particle size ratio.
The technique suggested in Japanese Patent Publication (Koukai) 4-54494 entails overlaying a bed of coarse particles on an entraining bed of recirculating fine particles to create a low-speed region on top of a high-speed region. The aforesaid low-speed region of coarse particles has two gas inlets to insure that it remains completely fluidized. The speed and efficiency of the reaction can be adjusted by increasing or decreasing the velocity of the fluidizing gas and the recirculation rate of the fine particles.
Just how much the capacity of the system can be increased in the ways described above is limited by the size of the fine and coarse particles and by how well the coarse particles can be fluidized, which depends largely on the aforesaid speed of fluidization. There is also a tendency for changes in the system to result in unstable reaction conditions.
Since the device disclosed in Japanese Patent Publication (Koukai) 4-54494 also entails overlaying a dense bed of coarse particles on an entraining bed of fine particles, it, like the two inventions previously discussed, suffers from extensive abrasion of the coarse particles which serve as the fluidizing medium in the heavy bed. Its maintenance is complicated by the requirement that the coarse particles be replenished very frequently, and the use of coarse particles which are prone to abrasion results in variation in the particle size ratio, which causes the system to be unstable. Furthermore, even the fact that the device has two gas inlets results in virtually no better control of the suspension density of the fine particles in the entraining bed.
The following design has also been proposed for a fluidized bed incinerator and its drive method.
Japanese Utility Model Publication (Koukai) 61-84301 offers a design for a fluidized bed incinerator which has heat transfer pipes in the bed to conserve and redistribute heat within the system. These pipes are arranged in the bed so that their axes are at an angle between 0 and 15xc2x0 with respect to a perpendicular through the splash zone of the bed; in other words, they are virtually perpendicular.
The invention disclosed in Japanese Patent Publication (Koukai) 5-223230 comprises a fluidized bed combustion furnace in which a portion of the floor of the furnace, which portion is inclined at an angle of at least 10xc2x0, is perforated to form an air dispersion panel. The remainder of the bottom of the fluidized bed has air dispersion pipes in it. The fluidizing medium is poured onto these two portions of the floor, forming a fluidized bed with air dispersion tubes and an inclined fluidized bed with perforations to disperse the air, or a static bed. The fluidizing medium, as well as any uncombusted matter, is removed via pipe 17 on the floor of the furnace. Fluidizing medium of a specified particle size is recirculated and supplied to the inclined, perforated portion of the floor through an opening for that purpose. The garbage to be burned is also deposited on the inclined portion of the floor. A quantity of air which is from 0.7 to 1.5 times that of the minimum volume of gas required to fluidize the bed is supplied, and the garbage is gradually heated, disintegrated and combusted. A quantity of air which is from 2 to 9 times that of the minimum volume of fluidizing gas is supplied to the remaining char on the portion of the floor with the dispersion pipes, and it too is combusted. In this way, even if the quality of the fuel or the volume supplied should undergo a large momentary fluctuation, it will not result in incomplete combustion due to insufficient oxygen or the production of a large quantity of CO.
The invention disclosed in Japanese Patent Publication (Koukai)) 64-54104 comprises a fluidized bed combustion furnace. This furnace has a combustion tower in the bottom of which a layer of solid particles consisting of sand or ash is created and maintained; a mechanism in the middle of the layer of solid particles to inject a fluidizing gas in order to create a fluidized bed in the upper portion of the particle layer; a mechanism to cool the particles, which is placed in the static bed comprising the particle layer below the fluidized bed, and which cools the particles by means of heat exchange with water or air; a mechanism to recirculate the particles, which returns them to the fluidized bed via an exhaust port in the bottom of the tower; and a control mechanism, which controls the quantity of particles recirculated.
In the prior art designs disclosed in the aforesaid Japanese Utility Model Publication (Koukai) 61-84301, Japanese Patent Publications (Koukai) 5-23230 and 64-54104, there are no mechanisms to control precisely the ratio of primary and secondary air, to recirculate particles efficiently to the sand bed in order to absorb abnormal temperatures in the freeboard which are caused by load fluctuations or variation in the characteristics of the waste material, or to maintain the proper temperature in the sand bed.
Japanese Patent Publications (Koukoku) 59-13644 and 57-28046 offer designs which can be applied to this sort of fluidized bed incinerator and its operating method, but these, too, lack any means to address the problem areas described above.
To solve these problems, the first objective of the present invention was to provide a fluidized bed incinerator and an operating method for it which would increase the thermal capacity of the freeboard to respond to fluctuations of the load imposed by waste matter such as sludge or garbage with a high moisture content; which would absorb local and momentary temperature spikes due to load fluctuations or variations in the characteristics of the waste material; and which would recirculate the combustion heat generated in the freeboard and use it to maintain the temperature of the sand bed so as to reduce the need for accelerant.
The second objective of this invention was to provide a fluidized bed incinerator and an operating method for it which would enable the waste matter to be combusted in the deep portion of the fluidized bed. This portion extends as far as the bubbling region and the dense bed, which are below the surface of the bed of fluidized sand. In this way a greater quantity of waste material can be combusted in the sand bed, which has a higher thermal capacity than the freeboard.
Other objectives of this invention is disclosed in the following descriptions.
According to the invention disclosed in one embodiment, the fluidized bed incinerator has a splash region in which the particles of the fluidizing medium are propelled upward when the bubbles on the surface of the fluidized sand in the fluidizing region burst by injecting the primary air from the bottom of the fluidized bed for fluidizing the sand, and a freeboard region provided above the splash region, comprising: 1) an entraining region in which the particles are entrained and conveyed upward to the freeboard region by introducing the secondary air; 2) a recirculation unit to separate the particles of the fluidizing medium from the mixture of the exhaust gases and the fluidizing medium, and recirculate the fluidizing medium to the fluidizing region; and 3) an air control unit to adjust the ratio of the primary and secondary air based on the temperature difference between the freeboard region and the fluidizing region.
The air control unit preferably comprises a first damper to control the primary air to be introduced into the fluidizing region, and a second damper to control the secondary air to be introduced into the splash region, thereby said air control unit controls the ratio of the primary and secondary air.
The invention disclosed in another embodiment is an operating method to operate a fluidized bed incinerator. It comprises steps of: 1) injecting the primary air for fluidizing the fluidizing medium from a bottom of the fluidizing region; 2) injecting the secondary air into the splash region in which the bubbles on the surface of the fluidized sand burst and the particles are propelled upward when the bubbles are burst; 3) entraining and conveying the fluidizing medium upward and out of said incinerator via the freeboard; 4) recirculating the fluidizing medium to the fluidizing region; and 5) controlling the thermal capacity of the freeboard and the temperature of the fluidizing medium to be constant by controlling the ratio of the primary and secondary air.
The controlling step preferably controls the suspension density in the freeboard and the volume of recirculated fluidizing medium by controlling the ration of the primary and secondary air. The suspension density in the freeboard is preferably kept between 1.5 kg/m3 and 10 kg/m3.
With the invention described above, a splash zone, namely a space of discontinuous density resulting from the primary air tossing up particles of sand, is created between the freeboard in the upper part of the furnace and the bed region in the lower part of the furnace. In this invention, secondary air is brought into this splash zone. The particles of sand lifted into the splash zone on the primary air are entrained and conveyed into the freeboard along with the primary air. Increasing the quantity of particles held up in the region through which the sand travels increases the thermal capacity of the freeboard. In this way the system can respond to load fluctuations.
In this invention, the aforesaid particles which are entrained on the air (i.e., the particles tossed up by the primary air) are separated from the air by a cyclone or other separation means provided in a later stage of their travel. They are then sent back to the bed region by a recirculation unit provided downstream from the cyclone. This design allows the combustion heat from the freeboard to be applied to the cooler fluidizing medium in the bed region, thus helping maintain the temperature of the sand bed and reducing the need for auxiliary fuel for that purpose.
In other words, since it is necessary to keep the sand in the fluidizing region at a constant temperature, the fluidizing medium which has absorbed the combustion heat in the hotter freeboard is sent back to the cooler dense bed of the fluidizing region to supply heat to the sand of the bed. This insures that the exhaust gas is at the appropriate temperature, and it eliminates the need for extra fuel.
The thermal capacity of the aforesaid sand in the freeboard is a thousand times greater than that of a gas. It is thus well suited to mitigate temperature fluctuations in the freeboard caused by variations in the characteristics of the sludge which is being combusted. The use of this sand can eliminate inhomogeneous combustion due to load fluctuations and enable stable combustion to take place.
When a control unit adjusts the relative opening of two dampers, it adjusts the ratio of primary to secondary air in the fixed quantity of air supplied to the furnace. This controls the holdup rate of the sand used as the fluidizing medium in the area above the point at which the secondary air is admitted. The suspension density in the freeboard is adjusted so that it remains between 1.5 kg/m3 and 10 kg/m3. This insures that the thermal capacity of the freeboard can be increased or decreased as needed to respond to load fluctuations.
In this way, the quantity of primary air which serves as the fluidizing gas can be increased to expand the fluidized bed. The height of the sand surface and that of the splash zone, demarked by the highest point reached by a tossed particle of sand, can thus be increased by introducing more primary air. By increasing or decreasing the holdup rate of the fluidizing medium entrained by the secondary air above its inlet in the splash zone, we can adjust the suspension density of the freeboard through which the medium passes so that it is between 1.5 kg/m3 and 10 kg/m3.
This ability to maintain the temperature of the sand in the aforesaid bed region at its proper value enables us to design a furnace with a smaller floor area which can still handle the high moisture component of sludge. The sand can be fluidized with a smaller volume of air, and the volume of air beyond what is strictly necessary for combustion can be minimized. The furnace produces less exhaust gas, the quantity of auxiliary fuel can be reduced, and the fuel cost can be held down.
When the suspension density in the freeboard is excessive, or more specifically, when it exceeds the aforesaid range, the aforesaid control unit reduces the proportion of primary air and increases the proportion of secondary air going into the furnace. This reduces the quantity of medium thrown up from the bed region and so reduces the quantity of the said medium which is in circulation. Reducing the quantity of sand in circulation prevents abrasion of the device and reduces the cost of operating the blowers.
According to the invention disclosed in certain preferred embodiments, the fluidized bed incinerator has a splash region in which the particles of the fluidizing medium are propelled upward when the bubbles on the surface of the fluidized sand in the fluidizing region burst by injecting the primary air from the bottom of the fluidized bed for fluidizing the sand, and a freeboard region provided above the splash region, comprising: 1) an entraining region in which the particles are entrained and conveyed upward to the freeboard region by introducing the secondary air; and 2) a secondary air control means provided with an air supplying unit to supply the secondary air from one of a plurality of air inlets which are provided in the splash region vertically, said secondary air control means to control the open and close of said air supplying unit.
The invention disclosed above is preferably comprising as follows.
1) The fluidized bed incinerator further comprises: 1) a recirculation unit to separate the particles of the fluidizing medium from the mixture of the exhaust gases and the fluidizing medium, and recirculate the fluidizing medium to the fluidizing region; and 2) an air control unit to adjust the ratio of the primary and secondary air based on the temperature difference between the freeboard region and the fluidizing region.
2) The secondary air control means controls the open and close of the plurality of air inlets based on the temperature difference between the freeboard region and the fluidizing region.
The invention disclosed in certain preferred embodiments is related to the operating method to operate a fluidized bed incinerator. The method comprises steps of: 1) injecting the primary air for fluidizing the fluidizing medium from a bottom of the fluidizing region; 2) injecting the secondary air into the splash region in which the bubbles on the surface of the fluidized sand burst and the particles are propelled upward when the bubbles are burst, said secondary air being injected selectively from one or more air inlets provided vertically; 3) entraining and conveying the fluidizing medium upward and out of said incinerator via the freeboard; and 4) controlling the suspension density in the freeboard by selecting the air inlets for adjusting the height of said injecting the secondary air.
The following operation methods can be preferably added to the method disclosed above.
1) Recirculating the fluidizing medium via a recirculation unit provided out of the fluidized bed incinerator.
2) The controlling step controls the suspension density in the freeboard and the volume of recirculated fluidizing medium by controlling the ration of the primary and secondary air. The suspension density in the freeboard is preferably kept between 1.5 kg/m3 and 10 kg/m3.
With this invention, when the bubbles on the surface of the bubbling bed burst, some of the sand particles which constitute the fluidizing medium are tossed upward, forming a splash zone consisting of a layer of discontinuous density over the aforesaid bed region. A number of supply units for secondary air are provided at different heights in the splash zone, where particles of sand separated from the surface by air bubbles are floating about. Through one of these units, a control device for the secondary air selectively admits air at a given height. This creates an entraining region which extends as far as the freeboard above the splash zone. The particles of fluidizing medium are thus entrained and conveyed out of the furnace.
Since the freeboard, through which the particles of fluidizing medium are being entrained and conveyed, can hold up as many particles as reach it, this design greatly increases the suspension density in the freeboard as well as its thermal capacity. As a result, it is better able to respond to load fluctuations.
By admitting the aforesaid secondary air selectively through one of a number of supply units at different heights, we can adjust the suspension density in the freeboard above the point at which the air enters the furnace so that it remains between 1.5 kg/m3 and 10 kg/m3. More specifically, since the splash zone into which the supply units for the secondary air open is created when air bubbles on the bed surface burst, sending particles of sand flying up into the air, its density is highest immediately above the surface and decreases as the distance from the surface increases. Thus the density of the fluidizing medium entrained on the secondary air will be greater if the air is admitted closer to the surface. Admitting air through the lowest channel will yield the greatest suspension density in the freeboard.
Thus by selecting one of the various supply channels for secondary air which are provided at different heights in the furnace, we can adjust the suspension density of the sand particles carried to the freeboard by the secondary air. More specifically, by selecting an appropriate channel for the secondary air and an appropriate means to admit the air, we can adjust the suspension density in the freeboard so that it remains within its required range, between 1.5 kg/m3 and 10 kg/m3. This will allow the furnace to respond to sudden temperature spikes resulting from variations in the characteristics of the waste material.
With this invention, the particles of fluidizing medium (i.e., the particles thrown up by the air bubbles) entrained and conveyed as described above are separated from the air by a cyclone or other separator device placed downstream from the aforesaid entraining area. The particles pass through an external recirculation unit which includes the aforesaid separator device and are returned to the aforesaid bubbling region. In this way the combustion heat from the freeboard can be applied to the cooler fluidizing medium in the bubbling region so as to maintain the required temperature in the sand bed and thus reduce the need for auxiliary fuel for that purpose.
In other words, since it is necessary to keep the sand in the aforesaid fluidizing region at a constant temperature, the fluidizing medium which has absorbed the combustion heat in the hotter freeboard is sent back to the cooler dense bed of the fluidizing region to supply heat to the sand of the bed. This insures that the exhaust gas is at the appropriate temperature, and it eliminates the need for extra fuel.
The ratio of primary to secondary air determines what quantity of the aforesaid particles which are tossed up will be circulated. By adjusting this ratio, we can keep the temperature of the fluidizing region constant. By returning the fluidizing medium which has absorbed the combustion heat in the hotter freeboard to the cooler dense bed of the fluidizing region, we can supply heat to that region.
According to the invention disclosed in certain preferred embodiments, the fluidized bed incinerator comprises: 1) a splash region in which the particles of the fluidizing medium are propelled upward when the bubbles on the surface of the fluidized sand in the fluidizing region burst by injecting the primary air from the bottom of the fluidized bed for fluidizing the sand; 2) a freeboard region provided above the splash region; 3) an entraining region in which the particles are entrained and conveyed upward to the freeboard region by introducing the secondary air; 4) a recirculation unit to separate the particles of the fluidizing medium from the mixture of the exhaust gases and the fluidizing medium by a separation means, and recirculate the fluidizing medium to the fluidizing region; and the recirculation unit comprises: 4-1) a sealed pot provided under said separation means, said sealed pot comprising an accumulation region to accumulate the fluidizing medium separated by said separation means, and a pressurized region to recirculate the fluidizing medium into a connecting duct connected to the fluidizing region by the pressure of the recirculation air introduced from the bottom of said accumulation region; and 4-2) a recirculation control means to control the recirculation air in order to control the quantity of the fluidizing medium.
The fluidized bed incinerator preferably comprises an air control unit to adjust the ratio of the primary and secondary air based on the temperature difference between the freeboard region and the fluidizing region.
This invention comprises a fluidized bed incinerator for sewage sludge, municipal garbage, or other waste with a high moisture content. In this incinerator, the thermal capacity of the freeboard can be increased to respond to load fluctuations so that local or momentary temperature spikes due to load fluctuations can be absorbed. The combustion heat produced in the said freeboard is recirculated to help maintain the proper temperature in the sand bed, and the suspension density in the freeboard can be increased for the same purpose.
With this invention, then, primary air fluidizes a bed region and causes bubbles to form in it. When the bubbles on the surface of the bed burst, particles of sand are tossed upward to form a splash zone, a layer of discontinuous density over the aforesaid bed region. When secondary air is blown into this splash zone, groups of particles separated from the surface by the bursting of bubbles are entrained on the secondary air and conveyed through the freeboard and out of the furnace. The suspension density in the freeboard is adjusted by changing the quantity of particles entrained by the secondary air, which is accomplished by altering the ratio of the aforesaid primary to secondary air. A control unit also adjusts the total volume of primary and secondary air supplied to the furnace. The suspension density is controlled by the following means. An appropriate quantity of the sand entrained by the aforesaid secondary air and stored temporarily in an external recirculation unit is recirculated to adjust the holdup rate of the sand bed in the bubbling region. This results in an adjustment of the suspension density in the freeboard.
To be more specific, with this invention, the volume of air blown into the bottom of the recirculation segment of the aforesaid sealed pot is adjusted in order to cause the sand bed consisting of sand collected in the said recirculation segment to expand. The topmost layer of the expanded bed will overflow out of the sealed pot and return to the sand bed in the bubbling region. This will increase the holdup rate in the bubbling region, and as a result the holdup rate in the freeboard will also increase, resulting in a greater suspension density.
The control unit controls the ratio of primary to secondary air. By controlling this ratio, we can control the holdup rates in the bed region and the freeboard, which are in an inverse relation with each other, and the suspension density and quantity of particles in circulation in response to fluctuations of the combustion characteristics of the material to be incinerated.
If, for example, we increase the proportion of primary air, we will increase the quantity of particles tossed up from the bed region. This will increase the holdup rate in the space above the inlet for the secondary air. It will also increase the suspension density in the freeboard and the quantity of particles in circulation.
According to the invention disclosed in certain preferred embodiments, the fluidized bed incinerator comprises: 1) a splash region in which the particles of the fluidizing medium are propelled upward when the bubbles on the surface of the fluidized sand in the fluidizing region burst by injecting the primary air from the bottom of the fluidized bed for fluidizing the sand; 2) a freeboard region provided above the splash region; 3) an entraining region in which the particles are entrained and conveyed upward to the freeboard region by introducing the secondary air; 4) a recirculation unit to separate the particles of the fluidizing medium from the mixture of the exhaust gases and the fluidizing medium and recirculate the fluidizing medium to the fluidizing region; 5) a buffer tank to store the fluidizing medium discharged from an outlet along with uncombusted material, which is provided below the fluidizing region; and 6) a buffer tank control means to control the supplying the fluidizing medium to the fluidizing region based on the temperature in said freeboard region depending on the load fluctuation in said fluidized bed incinerator.
According to the invention disclosed in certain preferred embodiments, the fluidized bed incinerator comprises: 1) a splash region in which the particles of the fluidizing medium are propelled upward when the bubbles on the surface of the fluidized sand in the fluidizing region burst by injecting the primary air from the bottom of the fluidized bed for fluidizing the sand; 2) a freeboard region provided above the splash region; 3) an entraining region in which the particles are entrained and conveyed upward to the freeboard region by introducing the secondary air; 4) a recirculation unit to separate the particles of the fluidizing medium from the mixture of the exhaust gases and the fluidizing medium and recirculate the fluidizing medium to the fluidizing region; 5) a buffer tank to store the fluidizing medium discharged from an outlet along with uncombusted material, which is provided below the fluidizing region; 6) an air control unit to adjust the ratio of the primary and secondary air based on the load fluctuation in said fluidized bed incinerator; and 7) a buffer tank control means to control the supplying the fluidizing medium to the fluidizing region based on the load fluctuation.
The air control unit disclosed preferably controls as follows.
1) It adjusts the ratio of the primary and secondary air based on the temperature difference between the freeboard region and the fluidizing region, and said buffer tank control means controls the quantity of the fluidizing medium for providing to the fluidizing region based on the temperature at a predetermined location in said fluidized bed incinerator.
2) It adjusts the ratio of the primary and secondary air so that the sum of the quantities of primary air and secondary air remains constant.
With this invention, primary air fluidizes a bed region and causes bubbles to form in it. When the bubbles on the surface of the bed burst, particles of sand are tossed upward to form a splash zone, a layer of discontinuous density over the aforesaid bed region. When secondary air is blown into this splash zone, groups of particles separated from the surface by the bursting of bubbles are entrained on the secondary air and conveyed through the freeboard and out of the furnace. The suspension density in the freeboard is adjusted by changing the quantity of particles entrained by the secondary air, which is accomplished by altering the ratio of the aforesaid primary to secondary air. More specifically, the suspension density is adjusted to remain between 1.5 kg/m3 and 10 kg/m3. The fluidizing medium which has been discharged from the furnace via the outlet on the bottom of the fluidized bed is stored in a buffer tank. In order to achieve a wide range of suspension densities, these sand particles are supplied to the furnace as needed to respond to the state of the load. This constitutes an internal recirculation unit for the sand which allows the suspension density in the freeboard and the quantity of particles in circulation to be adjusted over a wide range of values.
To be more specific, the fluidizing medium is passed through a vibrating sieve or other separation device on the outlet for uncombusted material on the bottom of the fluidized bed. The filtered fluidizing material is collected in a buffer tank. In response to the state of combustion in the freeboard, an appropriate quantity of medium is supplied to the combustion chamber of the furnace, i.e., to the freeboard. In this way the holdup rate in the freeboard is adjusted and the suspension density and the quantity of particles in circulation is increased. A wide range of responses is thus available for load fluctuations.
With this invention, because the sand is kept circulating through the freeboard so that its thermal capacity is available to absorb temperature fluctuations which occur there, the temperature in the furnace can be kept constant despite load fluctuations, and the furnace can operate in a stable fashion. Because the hotter medium is returned to the dense bed, the sand in the bed can be kept at the required temperature, and the load consisting of moisture content on the floor of the furnace can be increased. This invention reduces the quantity of exhaust gas and the required fuel cost, and it insures that the exhaust gas will be at the required temperature.
Because the ratio of primary to secondary air is controlled, the holdup rates in the bed region and the freeboard, which are in an inverse relation with each other, can be adjusted in response to variations in the combustion characteristics of the material to be incinerated. To be more specific, the suspension density is kept between 1.5 kg/m3 and 10 kg/m3.
According to the invention disclosed in certain preferred embodiments, the fluidized bed incinerator comprises: 1) a bubble fluidizing region having a dense region and a bubbling region above said dense region; 2) a splash region in which the particles of the fluidizing medium are propelled upward when the bubbles on the surface of the fluidized sand in said bubble fluidizing region burst by injecting the primary air from the bottom of the fluidized bed for fluidizing the sand; 3) a freeboard region provided above the splash region; 4) an entraining region in which the particles are entrained and conveyed upward to the freeboard region by introducing the secondary air; 5) a recirculation unit to separate the particles of the fluidizing medium from the mixture of the exhaust gases and the fluidizing medium and recirculate the fluidizing medium to said dense region; and 6) a waste inlet through which the waste material is loaded, which is to be incinerated in said bubble fluidizing region having said dense region and said bubbling region.
The fluidized bed incinerator above preferably comprises a fluidizing medium inlet for returning said fluidizing medium placed at the same height as said waste inlet or at the lower position than said waste inlet, and an auxiliary burner.
With this invention, the waste material is introduced into the dense bed in the region which is fluidized by blowing in air. Combustion occurs in the deep portion of the fluidized bed, including the said dense bed and the bubbling region on top of it. The material is thus combusted in the sand bed, which has a high thermal capacity. This insures that stable combustion can be maintained.
The waste material is introduced directly into the very hot fluidized bed below the vigorously fluidized bubbling region, whose surface remains in a boiling state. The waste is pulverized when it experiences the explosive force of momentary volatilization of its moisture component and distributed uniformly throughout the entire bubbling region above the bed. Thus even the dense bed on the bottom of the bed region can be used efficiently for combustion. This results in a wider range of permitted loads.
Because the waste material is supplied to a relatively deep portion of the fluidized bed, only a small proportion of its volatile component is lost to the freeboard. The greater portion is combusted in the sand bed, which has a higher thermal capacity. This design allows the furnace to absorb load fluctuations and maintain a stable temperature.
As was discussed above, the waste material which is introduced into the middle of the fluidized bed, in an area which is fluidized at a high temperature and under extreme pressure, experiences the tremendous force produced by instantaneous volatilization of its moisture component. This prevents the formation of clods of melted ash which would impede fluidity.
Placing the inlet for medium being returned from the external recirculation unit and the installation for the auxiliary burner at the same level or lower than the inlet for the aforesaid waste material prevents the temperature of the fluidized bed from dropping when waste is loaded into the aforesaid dense bed.